1. Field of the Invention
This invention relates in general to a lifting device and in particular to a controlled passive porosity system used to promote fluid communication between a low pressure surface and a high pressure surface of the lifting device.
2. Description of Related Art
Cavitation generally occurs when the local pressure of a multi-phase liquid experiencing fluid flow drops below the vapor pressure of the liquid. When the pressure of the liquid at any given point drops below the vapor pressure, the liquid at that point will transform to a gas. Since pressure drops often occur in localized areas, the transformation of the liquid to a gas generally causes gaseous bubbles in the liquid.
Lifting devices such as boat propellers, submarine propellers, impellers, and fluid pump blades are used to transfer energy to a multi-phase liquid such as water, thereby imparting a fluid flow to the liquid relative to the lifting device. Lifting devices such as hydrofoils are used to generate lift when exposed to a multi-phase liquid having a fluid flow. Lifting devices such as hydro-power plant turbine blades are used to capture energy from a flowing multi-phase liquid. All of these lifting devices are characterized by a general shape, which when exposed to a fluid flow, experiences a high pressure region on a high pressure surface and a low pressure region on a low pressure surface.
The low pressure region of the liquid along the low pressure surface of the lifting device is especially prone to occurrences of cavitation. If the pressure of the liquid at any point along the low pressure surface drops below the vapor pressure of the liquid, the liquid will transform into gaseous bubbles. The presence of cavitation along any surface of a lifting device can be very harmful to the overall performance of the lifting device. Additionally, cavitation can produce large acoustic emissions and can cause severe erosion along the surfaces of the lifting device.
The specific problems caused by cavitation are costly, both in economic terms and in performance terms. An example of this can be seen in the use of turbine generator blades in hydroelectric dams. As water is driven through the turbine blades, cavitation occurs, which causes accelerated erosion to the surfaces of the blades. Periodic maintenance is required to xe2x80x9creplacexe2x80x9d the material that has eroded away. This is done by welding additional material to the blade and then grinding the blade down to its original shape.
Cavitation problems are also evident in ships and submarines which use propellers for propulsion. In both ships and submarines, the efficiency of power transfer is decreased due to cavitation. In submarines, cavitation produces increased acoustical emissions, which are detrimental to the stealthy operational requirements of the vehicle.
Cavitation is usually controlled through geometry optimization of the lifting device. Another method of controlling cavitation is to restrict the operational envelope in which the lifting device will work. Less frequently used techniques of cavitation control include air injection, polymer injection, and fixed porosity systems. Passive porosity systems have been used in aircraft applications to equalize the pressures between two areas.
Cavitation problems continue to exist when using lifting devices in a multi-phase liquid. Since cavitation can occur randomly due to the distribution of nuclei or micro-air bubbles and flow turbulence, geometric optimization or restricted operational envelopes will not insure cavitation free operation.
The controlled passive porosity system of the present invention solves the cavitation problems associated with the use of lifting devices in multi-phase liquids. The preferred embodiment of the invention includes a lifting device having an outer skin with a low pressure surface, a front surface, and a high pressure surface. A common plenum is located within the lifting device, the common plenum being located just beneath the low pressure surface, the front surface, and the high pressure surface. A plurality of holes are disposed within the low pressure surface and the high pressure surface to allow fluid communication between the space surrounding the lifting device and the common plenum. The presence of the holes in the low pressure and high pressure surfaces of the outer skin, coupled with the presence of the common plenum, makes possible fluid communication between the space adjacent to the low pressure surface and the space adjacent to the high pressure surface.
Preferably, a microelectromechanical valve and a sensor is associated with each hole in one of the surfaces (either the low or high pressure surface). The pressure sensors and valves allow individualized control over the holes, thereby enabling the passive porosity system to effectively control local occurrences of cavitation.
In normal operation, the lifting device encounters fluid flow of a multi-phase liquid such as water. As the liquid flows around the lifting device, a low pressure region generally develops adjacent to the low pressure surface and a high pressure region develops adjacent to the high pressure surface. To eliminate cavitation along the low pressure surface of the lifting device, the microelectromechanical valve located in that area is opened to allow fluid communication. The resulting fluid communication between the liquid adjacent to the high pressure surface and the liquid adjacent to the low pressure surface locally equalizes the pressures between the two surfaces. This pressure equalization raises the pressure of the liquid on the low pressure surface, thereby eliminating cavitation.
In an alternate embodiment, the lifting device has a first plenum located beneath the low pressure surface and a second plenum located beneath the high pressure surface. The upper and second plenums are fluidly connected by an intermediate fluid passage. In this embodiment, a valve can be located in the intermediate fluid passage to selectively block or allow fluid communication between the first and second plenums. Ideally, several first and second plenums would be disposed within each lifting device, with each first plenum being fluidly isolated from the other first plenums.
The operation of the alternate embodiment would be essentially the same as that of the preferred embodiment. As cavitation occurs on the low pressure surface, it would be eliminated by opening the appropriate valve. The valve would allow fluid communication between the liquid adjacent to the high pressure surface and the liquid in the area experiencing cavitation.